The US National Cancer Institute cut short its research after results showed the positive effects of tamoxifen

Scientists in Europe and Australia have insisted the breast cancer drug tamoxifen needs further tests, despite a US study showing beneficial results.

Doctors in Britain voiced concern that their American colleagues may have put research in Europe and Australia at risk by ending trials last month on 13,000 women in the United States prematurely.

The US National Cancer Institute (NCI) cut short its research after early results showed tamoxifen reduced the incidence of the disease by 45% in high-risk women.

Trials with tamoxifen are continuing on 12,000 women in Europe and Australia and researchers say major questions remain before the drug can be certified for general use.

The NCI study had been due to last until the year 2000, but the institute decided to allow participants in the study who had been given dummy pills to take tamoxifen instead because it was generally agreed to be effective.

Now, British doctors fear that thousands of women currently using placebos will withdraw from trials and demand tamoxifen instead.

Some women are already questioning why it is necessary to carry on with the trials and treating people with the placebo - knowing that will not cure them or stop the disease - if it is obvious that the drug is working.

British doctors co-ordinating the International Breast Cancer Intervention Study (IBIS) in the United Kingdom, Finland, Switzerland and Australia decided not to halt their project because there were still major questions to be answered.

"An enormous amount of effort has been put into this trial, and we owe it to the women participating in this study to ensure that we get clear answers about the risks and benefits associated with the use of tamoxifen for prevention," Dr Jack
Cuzick, the chairman of the IBIS working group, said in a statement.

They said the American study did not show whether the drug reduced death rates or just delayed the onset of breast cancer, and they also pointed to some side-effects.